Anthony Graham — Page 2

PubMed2002

Early steps in the production of sensory neurons by the neurogenic placodes.

Neurogenic placodes are specialized regions of the embryonic ectoderm that generate the majority of the neurons of the cranial sensory ganglia. Here we have accurately determined the onset of neurogenesis in each of the placodes in the chick, and we have also analyzed the expression profiles of genes that are believed to be involved in determining the types of sensory neurons produced by each placode. Interestingly, we find that there is a major difference in the expression domains of neurogenin-1 and neurogenin-2 in the chick, when compared with those reported for the mouse. We do find, however, that Brn-3a and Phox-2a and Phox-2b which are also associated with the specification of neuronal type are expressed in the same domains in the chick as they are in the mouse. These results suggest that neurogenin-1 and neurogenin-2 are functionally interchangeable in neurogenic placodes. We have also found major differences between the ophthalmic and maxillomandibular trigeminal placodes, and while all of the other placodes generate mitotically active cells the ophthalmic trigeminal placode seems to throw off postmitotic neuronal cells.

Animals↗

PubMed2002

Wnt6 marks sites of epithelial transformations in the chick embryo.

In a screen for Wnt genes executing the patterning function of the vertebrate surface ectoderm, we have isolated a novel chick Wnt gene, chick Wnt6. This gene encodes the first pan-epidermal Wnt signalling molecule. Further sites of expression are the boundary of the early neural plate and surface ectoderm, the roof of mesencephalon, pretectum and dorsal thalamus, the differentiating heart, and the otic vesicle. The precise sites of Wnt6 expression coincide with crucial changes in tissue architecture, namely epithelial remodelling and epithelial-mesenchymal transformation (EMT). Moreover, the expression of Wnt6 is closely associated with areas of Bmp signalling.

Amino Acid Sequence↗

PubMedDate not supplied

Gene deployment for tooth replacement in the rainbow trout (Oncorhynchus mykiss): a developmental model for evolution of the osteichthyan dentition.

Repeated tooth initiation occurs often in nonmammalian vertebrates (polyphyodontism), recurrently linked with tooth shedding and in a definite order of succession. Regulation of this process has not been genetically defined and it is unclear if the mechanisms for constant generation of replacement teeth (secondary dentition) are similar to those used to generate the primary dentition. We have therefore examined the expression pattern of a sub-set of genes, implicated in tooth initiation in mouse, in relation to replacement tooth production in an osteichthyan fish (Oncorhynchus mykiss). Two epithelial genes pitx2, shh and one mesenchymal bmp4 were analyzed at selected stages of development for O. mykiss. pitx2 expression is upregulated in the basal outer dental epithelium (ODE) of the predecessor tooth and before cell enlargement, on the postero-lingual side only. This coincides with the site for replacement tooth production identifying a region responsible for further tooth generation. This corresponds with the expression of pitx2 at focal spots in the basal oral epithelium during initial (first generation) tooth formation but is now sub-epithelial in position and associated with the dental epithelium of each predecessor tooth. Co-incidental expression of bmp4 and aggregation of the mesenchymal cells identifies the epithelial-mesenchymal interactions and marks initiation of the dental papilla. These together suggest a role in tooth site regulation by pitx2 together with bmp4. Conversely, the expression of shh is confined to the inner dental epithelium during the initiation of the first teeth and is lacking from the ODE in the predecessor teeth, at sites identified as those for replacement tooth initiation. Importantly, these genes expressed during replacement tooth initiation can be used as markers for the sites of "set-aside cells," the committed odontogenic cells both epithelial and mesenchymal, which together can give rise to further generations of teeth. This information may show how initial pattern formation is translated into secondary tooth replacement patterns, as a general mechanism for patterning the vertebrate dentition. Replacement of the marginal sets of teeth serves as a basis for discussion of the evolutionary significance, as these dentate bones (dentary, premaxilla, maxilla) form the restricted arcades of oral teeth in many crown-group gnathostomes, including members of the tetrapod stem group.

Animals↗